Electric melting furnace



Nov. 18, 1930. F. LIN'NHOFF 1,782,359

n ELECTRIC MELTING FURNACE Filed Feb. 7, 1929 whahah Patented Nov.` 18, 1930 UNITED STATES PATENT .o1-FICE FRANZ LINNHOFF, OF EBERSWALDE,

GERMANY, ASSIGN OR TO AJAX ELECTROTHERMIG CORPORATION, F AJAX PARK, EWING TOWNSHIP, NEW JERSEY, A CORPORATION 0E NEW JERSEY ELECTRIC MEETING EURNACE Application ledlebruary 7, 1929, Serial No.

My invention relates to electric melting furnaces, more particularly to electric furnaces for melting light metals, e. g. alumlmum.

The electric furnaces having been employed for melting light metals up to the present time are either induction furnaces" generating the heat immediatelyJ in the -melting material or resistance furnaces transmitl ting the heat to the melting material by conduction and radiation.

In induction furnaces a strong stirring force is exerted on the molten bath due to pinch-effect, and this force is greater in the ease of a molten bath of low specilic gravity. In consequence of the movement caused by this force themolten material absorbs oxygen from the air. The aluminium oxide produced constitutes not only a loss of charge but causes also serious difficulty, because it sinks to the bottom of the molten bath and is hard to remove from the furnace. Thereby interruptions in the working of induction furnaces may easily occur because the aluminium oxide deposits in the heating channels and, owing to its high electrical resistance and its high melting temperature, entirely or partially, interrupts the electric circuit formed by the melting material.

These difficulties are not to be eliminated by covering the melting bath with a salt crust as the electrically neutral salt will collect in the lower parts of the furnace and so the melting material in the upper parts will soon be exposed to the oxidizing action ofthe air.

With resistance furnaces the input is limited. Assuming constant conditions of conductivity and radiation it depends upon ,the dierence of temperature between the heating resistance and the external wall of the crucible as well as upon the radiating surface. Since for the melting of aluminium a high utilization of energy is necessary in order to avoid a too long melting period it is advantageous to construct-the furnace for a resistance and of 338,205, and in Germany February 11, 1928.

high input. As the difference of temperature between the heating resistance and external wall of the crucible is a function of the permissible temperature of the heating the temperature of the molten material, an increase of the input of the furnace can be reached by means only of enlarging the radiating surface ofthe heating resistance, i. e. by enlarging the furnace for the same weight of charge. This causes, however, a decrease of the thermal efficiency of the furnace.

The object of my invention ,is to provide an electric melting furnace which makes it possible to melt light metals, especially aluminium, with a good thermal efficiency and slight loss of metal.

The invention consists in an electric melting furnace provided with means for inductive heating as `@Well as with means for resistance heating. The heating means are arranged so that the inserted melting material is first exposed to induction heating and then. when it begins to melt, is subjected to resistance heating. As by induction heating it is possible to put much energy into the furnace, the charge can be heated quickly to the melting temperature. When the charge begins to melt the induction heating ceases to operate and the further heating of the meltln material to the casting temperature takes p ace by resistance heating. Then the movement of the bath is so slight that by a suitable -cover the metal can easily be preserved from absorbing oxygen out of the air.

It is to be understood that instead of a furnace with two kinds of heating, two furnaces ma be used, one of them being an inductive fiirnace and the other a resistance furnace.- In this case the material to be melted is inserted in the induction furnace and the molten'metal subsequently will be led over into the resistance furnace in which AA preferred embodiment of my invention is shown diagrammatically in the accompanying drawing. 7

The crucible A is surrounded at its upper part by a coil B of copper strips or copper tubing which can be connected to an alternating current source at normal or high frequency. The lower part of the crucible is surrounded by heating resistances C whichmay be connected to any suitable current supply., Between the crucible and the coil B a layer D of heat insulating material and an electrical insulating cylinder E are placed. The heating resistances C are arranged in grooves within the refractory furnace lining F which -is surrounded by a heat insulating layer Gr and a heat insulating cylinder H of fulgurite, steatite or the like. The hearth level consists of a plate of metal as for example, iron, while the roof K as well as the conical cover piece L are made of chamotte or the like. The furnace may be emptied through the tap tube M or, in case the furnace is tiltably mounted, through a casting lip provided at the upper part of the furnace. The

windings of the coil B are pressed together by the press-plates N and the screws O. The screws O also hold the upper part andthe lower part of the furnace together.

The coil B can be switched off and on by the switch P. In order to decrease the input of the furnace the coil B may also be switched on partially; for thispurpose the coil B can be provided with one or moretaps. The heating resistance C can be switched olf and on by the switch Q. This switch also makes it possible to connect the heating resistance C to a part of the windings of the coil B and thus diminish the input of the heating resistance.

The switching over from induction heating to resistance heating may take place by hand or automatically depending upon the temperature of the melting material. At the furnace a pyrometer R is provided, being connected in known manner either to an indicating instrument or to a relay which controls the switches P and Q.. In the latter case the relay effects. the switching over of the heating devices as soon as the adjusting temperature of the melting material will 'be reached. l

It will be evident that I secure all of the advantage of an electrical'inductance heater through that part of the charge and throughout that part of the operation in which the otherwise inevitable pinch stirring eect is not operative and is, therefore, not objectionable because there is no con siderable body of molten material in which the pinching can take place; and that the heating resistance is effective as a resistor merely for the heat developed within its own metal with a very much slighter inductive effect-in comparison to the amount of heating than is present from the inductor coil above.

It will further be evident that the shoulder S of the upper part of the furnace is effective asa shelf to hold the major part of the charge, the central portion of the charge being sustained by such wedging of the contents as takes place. The secondary current induced bythe primary inductor B will be vgreatest in the outer part (radially) of the charge because of skin effect.

Having now particularly described and ascertained the nature of myinvention and in what manner it is to be performed, wliat I claim is:

1. In an .electric melting furnace, in combination, a crucible, an induction coil surrounding said crucible, heat insulation between the Crucible and the coil, a heating resistance arran ed on said crucible, a heat insulation outsi e of the heating resistance and confining its heat to the furnace.

2. In anelectric melting furnace, in combination, a crucible, an induction coil surrounding the upper part of said crucible, heat insulation between the crucible and the coil, a heating resistance surrounding the lower part of said crucible and means for confining the lheat ,from the heating resistance to the lower part of the furnace.

3. In an electric melting furnace, in combination, a crucible, an induction coil and a heating resistance, separate power connections for the coil and the resistance and means for connecting induction coil andresistance in series.

4. In an electric melting furnace, in combination, a crucible, an induction coil surroundingthe upper part, a heating resistance surroundingl the llower part of said crucible, said coil being provided with tappings and means for connecting the coil thus made adjustable with the heating resistance.

5. In an electric melting furnace, in combination, a crucible having a larger diameter in thel upper part of the crucible than Y in its lower part, an induction coil. surrounding the upper part and a heating resistance surrounding the lower part of'said crucible, said coil being provided with tappings, said coil and said heating resistance being connected to an electric current supply by switches which are adapted to switch over the heat resistance to a part of the induction coil. v

6.In an electric furnace, a furnace shell having upper and lower charge receiving portions, heat insulation about the upper portions, a furnace inductor about the heat insulation, connections for supplying current to the inductor, a furnace resistor surround-v ing the lower portion of the furnace having relatively high resistance and corresponding-V ly low inductive effect, heat insulation about the resistor confining the heat ofthe resistor against the furnace shell and connections for supplying electric current to the resistor.

7. In an electric furnace, a furnace body having charge receiving space of greater (lir anleter in the upper portion of the furnace than in the lower portion thereof, an electric inductor about the upper portion of the furnace, an annular shelf between the upper and lower portions and a furnace resistor in the space about the lower portion of the furnace. r

In testimony whereof I affix my signature.

FRANZ LINNHOFF. 

